A typical reduction gear of the prior art is that shown in FIGS. 11 and 12, which utilizes balls as power transmission elements. The assembly includes reduction gear 20 constituted of a drive shaft 22 having a cam 21 with an eccentric distance e. The assembly further includes a ball type Oldham's coupling 23, a ring-like eccentric plate 25 formed with a hypocycloid curve groove 24 on a cam 21.
The assembly further includes a driven disk 27 including an epicycloid curve grooved 26 having waves in number less than that of the hypocycloid curve by an even number and a driven shaft 28 is integrated with the driven disk 27. The eccentric plate 25 is opposed to the driven disk 27 through balls 29.
In operation therefore, when the drive shaft 22 is rotated, the eccentric plate 25 revolves tracing or depicting a circle having a radius e by the cam 21 in such a state as to be restricted in its rotation by the Oldham's mechanism 23. Thus, the eccentric plate 25 performs the eccentric motion. When the eccentric plate 25 revolves by the number of the waves of the groove 24, the driven disk 27 is rotated by the number of the above waves by the groove 26 and the balls 29.
The hypocycloid curve means the locus depicted by one point on the circumference of a rotating circle revolving along the inner side of a reference circle. The epicycloid curve means the locus depicted by one point on the circumference of the rotational circle revolving along the outer side of a reference circle.
The prior art differential reduction gears have certain disadvantages and drawbacks. For example, when an output shaft is applied with a thrust load coming from a leftward direction, as shown in FIG. 11, the balls and the groove are applied with a pressing force is exerted on the balls and groove of a sufficient magnitude to prevent the generation of a backlash between the balls and the groove to increase the Hertzian contact stress whereby the balls and groove are susceptible to damage and severe abrasion.
On the other hand, when an output shaft is applied with a thrust load coming from the rightward direction as shown in FIG. 11, the balls and the groove are separated from each other producing generated backlash therebetween.